That’s probably one of the main benefits of Sealion. I’ve found it usually is only feasible if the UK makes errors, as it’s quite easy for them to defend and quite hard to reinforce the Italian troops. Generally an Italian capture of South Africa only really works if the Axis control Egypt as the Italian bomber can reach it from there on the same turn as the attack (otherwise, you’ll have to take two turns to move the bomber from Tobruk to Ethiopia or somewhere that they can attack South Africa from). Capturing South Africa and producing from there makes it much harder for the Allies to retake Egypt and the rest of North Africa, buying the Axis valuable time.
Again, though, it’s unlikely a good UK player would allow such an opportunity to occur for the Italians. Even without any purchases in the region, they start off with enough units to blunt or delay an Italian advance (especially if you move units from India) long enough to allow American reinforcements (assuming it’s KGF).
